Which sea has no land coastline, its borders defined only by ocean currents?

The Sargasso Sea, located in the middle of the North Atlantic Ocean, is unique among the world's seas because it is bordered entirely by ocean currents rather than any land coastline. Its boundaries are formed by the North Atlantic Gyre, a system of rotating currents that traps large mats of free-floating sargassum seaweed within it, giving the sea its name and its golden-brown rafts of vegetation.

The Dead Sea and the Caspian Sea are both landlocked bodies of water surrounded by actual coastlines and countries, the opposite kind of case, so neither fits the description of a sea with no coastline at all.

The Sargasso Sea's calm, current-bound waters and abundant floating seaweed make it an important nursery habitat for eels, sea turtles, and various fish species, some of which travel enormous distances across the Atlantic specifically to spawn or shelter among its drifting sargassum mats.